An apparatus of this kind is known from the patent documents GB 2 061 746 (CA 1,177,061) or U.S. Pat. No. 4,211,277. The mixer element of this apparatus has a structure built up from webs which criss-cross each other and engage into one another wherein the webs form two groups of parallel structure elements.
A problem is caused by the webs which are arranged at the edge of the mixer element and contact the pipe wall at their outer edges or which at least lie in proximity thereto. More specifically, the medium to be mixed flows substantially more slowly in the region of these webs than in the inner region of the mixer element. In the corner region between the webs positioned at the edge and the pipe wall the medium thus has a relatively long dwell time which, as has been shown in practice, can lead to a drop in quality of the product produced from the mixed media.